A complaint of money laundering has been filed against Auckland Mayor Len Brown.

Self-professed anti-corruption campaigner Penny Bright, who contested the mayoralty against Brown at last year's election, laid the complaint at the Auckland central police station today, citing section 243 of the Crimes Act and alleging Brown concealed "freebies" from hotels across the city.

Bright alleged Brown had an interest in SkyCity, which provided him with free room upgrades.

"[Brown] did not declare nearly $40,000 ... effectively he concealed this information," she said."One way or another, Len Brown will be held to account."

An independent review by Ernst & Young found that Brown received 64 room upgrades, valued at $32,888, which were not registered as gifts or disclosed in his annual declaration of interests.

Auckland police spokeswoman Noreen Hegarty confirmed this afternoon that a complaint had been laid.

"That complaint will be assessed," she said.

Brown could not be reached for comment this afternoon.

Last month, a private prosecution against Brown, in which Bright was to be called to give evidence, struck a hitch in the Auckland District Court.

Documents filed by retired accountant Graham McCready in the court were sent back to him because he had failed to get leave from the attorney-general for the action.

McCready alleged that Brown failed to declare five free room upgrades from SkyCity for himself and his wife between November 2010 and November 2013. The upgrades were thought to be worth about $4500.

McCready alleged Brown failed to declare the gifts, which led to "favourable consideration" towards SkyCity, and he is attempting to prosecute the mayor under section 105(1) of the Crimes Act - corruption and bribery of an official.

Judge Ema Aitken ruled that before any official could be prosecuted under that section, leave must be sought by the attorney-general.

The attorney-general had not consented to the prosecution.

McCready said he would make an application to the attorney-general for consent, and if it was received, the private prosecution would be refiled.